The present invention is related to photoreceptors. In particular, the invention is related to photoreceptive materials which are sensitive to wavelengths up to .lambda..about.0.9 .mu.m.
The spectral response of photoreceptors is very important in determining the type of imaging system in which they can be used. Their overall performance is determined by their spectral sensitivity, or bandgap, and their electronic properties. For example, in systems which utilize light emitted by GaAs lasers or photodiodes it is necessary to have a photoreceptor with a large sensitivity to light of wavelength &gt;0.85 .mu.m. This requires materials with bandgaps which are &lt;1.45 eV. The high sensitivity beyond 0.7 .mu.m also improves the performance of the photoreceptor when used in conjunction with solid state lasers and photodiodes emitting light at .lambda..about.0.8.
Amorphous silicon passivated by hydrogen (a-Si:H) or other elements such as fluorine have been used as photoconductive receptors, electrophotography and image pickup tubes, see, e.g., U.S. Pat. No. 4,394,426, Maruyama et al, Journal of Non-Crystalline Solids 59 & 60 (1983) 1247-1254, North Holland Publishing Company and Imamura et al, Proceedings of the 11th Conference (1979 International) on Solid State Devices, Tokyo, 1979; Japanese Journal of Applied Physics, Volume 19 (1980) Supplement 19-1, pp. 573-577. However, because it has a bandgap of .about.1.7 eV, it has low optical absorption for .lambda..gtoreq.0.7 .mu.m and hence rapidly loses its photosensitivity at these wavelengths. To increase optical absorption of a semiconductor it is necessary to decrease its bandgap and there have been attempts to obtain amorphous semiconductors that have smaller bandgaps and good electronic properties. Amorphous semiconductors based on a-Si:H exist with smaller bandgaps such as a-Si.sub.x Ge.sub.1-x :H alloys in which a bandgap of .about.1.5 eV is obtained with .about.50% of Ge present in the films. In order to decrease the bandgap, E.sub.g, below 1.5 eV it is necessary to have more than .about.50% Ge in the films.
However, it is found that when the fraction of Ge exceeds .about.50% by volume and when E.sub.g decreases below .about.1.5 eV there is a large degradation in the electronic properties of the alloy films, see, e.g., G. Nakamura et al Japan J. Appl. Phys. 20, 1981, Suppl. 20-1, p. 291-296; R. L. Weisfield, J. Appl. Phys., 54, (1983) 6401. This degradation seriously limits any potential performance of a-Si:H/a-Ge:H alloys as photoreceptors having high sensitivity to wavelengths from 0.8 to 0.9 .mu.m.
In a preferred embodiment of the present invention, a photoreceptor member is described which includes material whose bandgap can be selected to make it sensitive to infrared light. The bandgap of the photosensitive material can be reduced below 1.5 eV with a ratio of Ge to Si which is .ltorsim.50% Ge and with sufficiently good electronic properties to make it highly photosensitive to wavelengths up to 0.9 .mu.m.